1. Field of the Invention
This invention relates to emission control of internal combustion engines. In particular, the invention relates to on-board monitoring of a primary fuel control oxygen (lambda or EGO) sensor for malfunction.
2. Prior Art
Various government regulations provide test conditions and monitoring methods for on-board monitoring of emission related devices. With respect to EGO sensor malfunction monitoring, the regulations suggest monitoring output voltage and response rate of the EGO sensors. Response rate is a time required for the EGO sensor to switch from lean to rich once it is exposed to a richer than stoichiometric exhaust gas or vice versa. A response rate check evaluates the portions of the sensor dynamic signal that are most affected by EGO sensor malfunctions such as aging, poisoning, or manufacturing deficiencies.
A deficiency of such methods is that during the test the fuel control system operates in an open loop control mode, and not a closed loop control made. As a result, air/fuel ratio tends to shift away from stoichiometry. Further, the particular selected air/fuel ratio swing and frequency greatly influence results of the test. The problem and disadvantages discussed above are overcome by this invention.